Cabbages mainly include Brassica campestris L. ssp. Pekinensis and Brassica campestris L. ssp. chinensis. Brassica campestris L. ssp. chinensis is also named as green cabbage, and baby Brassica campestris L. ssp. chinensis in the north of China. Brassica campestris L. ssp. chinensis exhibits high adaptability, growth, productivity and nutrition. It is the most consumed vegetable among various vegetables and widely grew in the provinces in the regions of Changjiang valley in China. There are various types and varieties of Brassica campestris L. ssp. chinensis. Cabbages have a short growth period, wide adaptability, and high productivity. They are also easy to plant, which allows for a sustained perennial supply.
The products of Brassica campestris L. ssp. chinensis are fresh and tender, have rich nutrition and win favor of consumers. Brassica campestris L. ssp. chinensis comprises about 30-40% of the total domestic vegetable productivity a year, and also makes a significant contribution in supplementing vegetables in slack seasons and balancing the vegetable supply over a whole year. Both the Brassica campestris L. ssp. Pekinensis and Brassica campestris L. ssp. chinensis favor cool weather and can be planted perennially. The most suitable growth temperature is 15-20° C. In recent years, to meet the market demand, cabbages are mainly planted by the technique of intensive culture. To ensure an even production and supply among the four seasons, Brassica campestris L. ssp. chinensis generally needs to be planted in different manners in different seasons. In the past, Brassica campestris L. ssp. chinensis was mainly planted in spring and winter. Now people begin to plant Brassica campestris L. ssp. chinensis in torrid summer and autumn by various culture manners. This will undoubtedly make Brassica campestris L. ssp. chinensis subject to the stress from drought during its growth, especially in late spring, summer and early autumn.
The Brassica campestris L. ssp. chinensis cultured in the seasons of high temperature can go to the market in bulk after a 20-day culture. However, the high temperatures usually lead to an elongated internode, slowed growth, bitter taste and undesirably increased fiber, etc. This will result in low productivity and poor quality. As a result, the price rises and the supply falls short of demand. The consumer demand cannot be met. Brassica campestris L. ssp. Pekinensis has poor tolerance to drought. It is highly drought sensitive in the rosette stage and the heading stage. If the average temperature is too high, the heart leaf can not amplexate to build a tight bulb, or can not bulb up at all. Even if it constrainedly bulbs up, the heading is loose. In the natural field conditions in summer, the production relies on the drought-resistance plants' capability of forming a normal leafy head. And the capability of heading formation under the natural high temperature in fields becomes an indication of a drought-resistance in Brassica campestris L. ssp. Pekinensis. 
Both the Brassica campestris L. ssp. Pekinensis and the Brassica campestris L. ssp. chinensis were originally planted in China. In foreign countries, there is few studies on breeding of cabbages. Varieties of Japanese, Korean and Formosan origins are poor in drought resistance, and unsuitable for planting in China. Domestically dominant are mainly the disease resistant varieties planted in autumn. Vegetables of cabbages have a narrow gene library for drought-resistance. Breeding of drought-resistance cabbage variety is limited to the screening among the cabbage materials, whereby only some varieties with poor drought resistance and low stress resistance have been obtained.
To solve these problems, the domestic breeding experts have utilized the traditional breeding methods to widely screen and culture drought-resistance varieties of vegetables of cabbages, to introduce drought-resistance genes, and broaden the sources of exploitation, which improved the drought-resistant ability of vegetables of cabbages to a certain degree and have produced effect in actual production. However, the current methods are limited to the assessment of drought resistance under the local climate and the morphological changes under a high temperature stress. These methods are not suitable for the temperate areas, which can not provide the field conditions with suitable selection stresses. Even if a single drought-resistance plant was selected, a series of complicated methods and means would be required to maintain the drought-resistance in the seeds collected until the next spring. The screening requires a long period, and is geographically limited, which can not provide a drought resistant variety universally adaptable. Therefore, it is an urgent task in breeding of drought-resistance vegetables of cabbages to intensively study the occurrence and development of the drought damages during the seedling stage, and to develop a method and technique for screening drought resistance in seedling stage, which provides improved operability, stability, efficiency and adaptability. The traits closely associated with the drought resistance in cabbages are of a quantitative nature, which poses great difficulties in genotyping. Particularly for molecular breeding, the difficulties include not only the limited number of DNA markers useful in the auxiliary selection, but also the inconsistence of the number and the significance of the quantitative traits loci (QTL). Therefore, since the genome sequencing of cabbages is not finished yet, and the study on functional genome study is gaining increasing interests, there is a need for a quick, sensitive and efficient qualitative analysis on the various traits in plant and the DNA profiles, and a quantitative analysis on the phenotypes in plant and changes in gene expressions, which is usefully in the breeding of drought-resistance cabbages.
There is a need in the art for identifying plant drought-resistance genes.